Ladder-type filter and multiplexer

Abstract

A ladder-type filter includes a support substrate, a piezoelectric layer provided on the support substrate, a parallel resonator including first electrode fingers provided on the piezoelectric layer and having a first average pitch and a first average duty ratio, a largest first average pitch being equal to or greater than two times a thickness of the piezoelectric layer, a first end of the parallel resonator being coupled to a path between input and output terminals, a second end of the parallel resonator being coupled to a ground, and a series resonator connected in series between the input and output terminals, the series resonator including second electrode fingers provided on the piezoelectric layer and having a second average pitch and a second average duty ratio, a second average duty ratio in at least one series resonator being less than a smallest first average duty ratio.

What is claimed is: 
     
       1. A ladder-type filter comprising:
 a support substrate; 
 a piezoelectric layer provided on the support substrate; 
 one or more parallel resonators each including a plurality of first electrode fingers provided on the piezoelectric layer, the first electrode fingers having a first average pitch and a first average duty ratio, a largest first average pitch being equal to or greater than two times a thickness of the piezoelectric layer, a first end of each of the one or more parallel resonators being coupled to a path between an input terminal and an output terminal, a second end of each of the one or more parallel resonators being coupled to a ground; and 
 one or more series resonators connected in series between the input terminal and the output terminal, each of the one or more series resonators including a plurality of second electrode fingers provided on the piezoelectric layer, the second electrode fingers having a second average pitch and a second average duty ratio, the second average duty ratio in at least one series resonator being less than a smallest first average duty ratio, 
 wherein the one or more series resonators are a plurality of series resonators, the at least one series resonator includes a first series resonator having a largest second average pitch; 
 wherein a second series resonator among the series resonators has a second average pitch less than the second average pitch of the first series resonator; and 
 wherein the second series resonator has a second average duty ratio greater than a second average duty ratio in the first series resonator. 
 
     
     
       2. The ladder-type filter according to  claim 1 , wherein the piezoelectric layer is a 36° or greater and 48° or less rotated Y-cut X-propagation lithium tantalate layer. 
     
     
       3. The ladder-type filter according to  claim 2 ,
 wherein no dielectric film covering the first electrode fingers and having a thickness greater than thicknesses of the first electrode fingers is provided on the piezoelectric layer in each of the one or more parallel resonators, and 
 wherein no dielectric film covering the second electrode fingers and having a thickness greater than thicknesses of the second electrode fingers is provided on the piezoelectric layer in each of the one or more series resonators. 
 
     
     
       4. The ladder-type filter according to  claim 1 , wherein each of the second average duty ratios in the one or more series resonators is less than the all of the first average duty ratios in the one or more parallel resonators. 
     
     
       5. The ladder-type filter according to  claim 1 , wherein each of the second average pitches in the one or more series resonators is less than all of the first average pitches in the one or more parallel resonators. 
     
     
       6. A ladder-type filter comprising:
 a support substrate; 
 a piezoelectric layer provided on the support substrate; 
 one or more parallel resonators each including a plurality of first electrode fingers provided on the piezoelectric layer, the first electrode fingers having a first average pitch and a first average duty ratio, a largest first average pitch being equal to or greater than two times a thickness of the piezoelectric layer, a first end of each of the one or more parallel resonators being coupled to a path between an input terminal and an output terminal, a second end of each of the one or more parallel resonators being coupled to a ground; and 
 one or more series resonators connected in series between the input terminal and the output terminal, each of the one or more series resonators including a plurality of second electrode fingers provided on the piezoelectric layer, the second electrode fingers having a second average pitch and a second average duty ratio, the second average duty ratio in at least one series resonator being less than a smallest first average duty ratio, 
 wherein a difference between a largest second average duty ratio and a smallest second average duty ratio is greater than a difference between a largest first average duty ratio and the smallest first average duty ratio. 
 
     
     
       7. A ladder-type filter comprising:
 a support substrate; 
 a piezoelectric layer provided on the support substrate; 
 one or more parallel resonators each including a plurality of first electrode fingers provided on the piezoelectric layer, the first electrode fingers having a first average pitch and a first average duty ratio, a largest first average pitch being equal to or greater than two times a thickness of the piezoelectric layer, a first end of each of the one or more parallel resonators being coupled to a path between an input terminal and an output terminal, a second end of each of the one or more parallel resonators being coupled to a ground; 
 one or more series resonators connected in series between the input terminal and the output terminal, each of the one or more series resonators including a plurality of second electrode fingers provided on the piezoelectric layer, the second electrode fingers having a second average pitch and a second average duty ratio, the second average duty ratio in at least one series resonator being less than a smallest first average duty ratio; and 
 a temperature compensation film provided between the support substrate and the piezoelectric layer, the temperature compensation film having a temperature coefficient of elastic constant opposite in sign to a temperature coefficient of elastic constant of the piezoelectric layer. 
 
     
     
       8. A multiplexer comprising:
 the ladder-type filter according to  claim 1 . 
 
     
     
       9. The ladder-type filter according to  claim 1 ,
 wherein the one or more parallel resonators have a pair of comb-shaped electrodes including the plurality of first electrode fingers, and reflectors, 
 wherein the one or more series resonators have a pair of comb-shaped electrodes including the plurality of second electrode fingers, and reflectors.